Dynamically Stable Rafts of Jumping Springtails

Springtails are millimeter-sized arthropods known for their impressive jumping abilities and their dynamic multitudinal aggregations at the surface of water. While biological collectives such as fire ants' rafts have been shown to cluster based on surface tension effects (attraction) and random walks (repulsion), the stability of springtails’ interfacial grouping remains unclear. Two distinct groups of collembolans, snow and marine springtails, have been observed to form floating rafts, but only the former have a functional jumping organ (i.e., the furcula). We discover that low-density rafts of snow springtails are rapidly destroyed by jumping individuals, breaking the Cheerios effect, while high-density rafts remain dynamically stable as jumpers leaving the raft are replaced by others. In contrast, marine springtails form stable rafts even at low densities due to their limited leaping and waking capabilities. Curiously, snow springtails are unable to jump with their tiny furcula directly from the water’s surface. Nevertheless, they can achieve a jump by climbing onto another’s body, generating a reaction force that propels the lower individual in the opposite direction, overcoming the Cheerios attraction. These results may inform the development of groups of robots capable of dynamically assembling and disassembling rafts on the surface of water.